This invention concerns a device for supplying a load with electric power, said power supply device comprising integrated energy storage.
Some energy consumers require a supply of predetermined electrical power as well as the supply of high power for a relatively short part of the cycle. Such cycles may be repeated. Examples of this type of consumer are the electromagnets of particle accelerators, rolling mill drives, lifts and elevators, cranes, etc. The direct connection of such consumers to the electrical power grid can cause serious grid disturbances. Devices are thus generally installed between the grid and the consumer to store energy so that it can be delivered at the required moment, thus avoiding peaks in power requirements being met directly by the power grid.
In addition, common energy supply devices are generally fitted with resistive energy dissipators that dissipate energy when the power flux is reversed, which obviously wastes a lot of energy.
Energy storage devices include storage systems for mechanical energy, for example the kinetic energy of a flywheel, storage systems for electrostatic energy, for example in capacitors and supercapacitors, storage systems for magnetic energy, for example in storage chokes, and storage systems for electromechanical energy, for example in accumulator batteries.
In particular, the article “A supercapacitor-based energy-storage system for elevators with soft commutated interface” by A. Rufer, published in “IEEE Transactions on Industry Applications”, vol. 38, No. 5, September/October 2002, describes an application for a lift or elevator in which power smoothing is achieved through a combination of a static converter and a variable-speed drive system. In this instance, the static converter allows a continuous bidirectional variation of the power flow using an intermediate DC circuit, which, in turn, is powered by a rectifier circuit located between the DC circuit and the primary AC grid. The article describes a means of providing the system with instantaneous power through a configuration comprising not only drive, static converter and rectifier components but also a storage device in the form of a bank of supercapacitors connected to an intermediate DC circuit using an additional static converter.
The U.S. Pat. No. 5,638,263 describes a similar device in which each DC/DC converter is supplied via its own voltage rectifier, which leads to a system which is clearly more expensive and clearly less reliable.
One of the aims of this invention is thus to propose an electric power supply device in which an energy storage device is positioned within the intermediate DC circuit(s) itself/themselves, obviating the need for an additional static converter. In the first instance, this eliminates power flows to and from an additional converter, thus improving the energy efficiency of the power device and reducing operating costs.
Another aim of the invention is to propose a bidirectional static converter device capable of satisfying a consumer's intermittent high power requirements and for which the necessary energy is taken directly from the intermediate DC circuit(s).
The discharge of energy from the intermediate DC circuit(s), which mainly comprise a series/parallel combination of capacitors, causes a variation in the level of DC voltage. This variation corresponds to a decrease in voltage when energy is drawn and an increase in voltage when energy is returned. To supply the consumer, e.g. the drive machine, under acceptable conditions, the capacitors of the intermediate DC circuit(s) must be adequately sized and voltage fluctuations of the DC circuit must be corrected eventually by modifying the modulation rate imposed on the static converter between the DC circuit and the consumer.
In the specific context of the invention's application to the power supply of the Proton Synchrotron at CERN in Geneva, the goal is to supply the consumer, in this case the magnet coils, with a high level of voltage, of the order of 10 kV. To this end, one of the aims of the invention is to propose a power supply device comprising several static converters placed in series, each including an intermediate DC capacitive circuit serving as a storage element. In a case like this, only some of the converters are supplied from the primary grid. The other converters/storage devices also have an intermediate DC capacitive circuit that is discharged or charged as energy is supplied to the magnet coils or recovered from them.
In such an application, current flows in the converters, the magnet coils and other components results in energy losses that cause a slow discharge of the capacitive circuits from one power cycle to the next. To compensate this discharge, another aim of the invention is to propose a device for controlling and adjusting the operation of all the converters so that energy can be transferred from one converter to another while the delivered current is flowing, thus allowing the mean values of the voltages of the intermediate DC circuits to be maintained by transferring energy from one converter to another, in particular from a powered converter to an unpowered converter.
To achieve this, power supply device of a load with the features mentioned in claim 1 is proposed, with embodiments as described in the dependent claims.
The following description concerns one embodiment of the invention, as applied to the realisation of a power supply for the Proton Synchrotron at CERN in Geneva. This description is provided merely as an example of how the invention can be realised, as many other applications are also possible. Numerical values are given for illustrative purposes only and refer to this particular energy consumer. The description below must be read with reference to the annexed drawing, which comprises the following figures.